Steam reforming and partial oxidation are widely practised and are used to produce hydrogen streams and synthesis gas for a number of processes such as ammonia, methanol production as well as the Fischer-Tropsch process. In a steam reforming process, a desulphurised hydrocarbon feedstock, e.g. methane, natural gas or naphtha, is mixed with steam and passed at elevated temperature and pressure over a suitable catalyst, generally a transition metal, especially nickel, on a suitable support. Steam is often provided by means of a saturator where water is contacted with the pre-heated hydrocarbon feedstock. For applications such as Fischer-Tropsch synthesis, it is desired that the molar ratio of hydrogen to carbon monoxide in the resulting synthesis gas is about 2 and the amount of carbon dioxide present is small.
Hence, in order to obtain a synthesis gas more suited to Fischer-Tropsch synthesis, the steam reformed gas may be subjected to partial combustion using a suitable oxidant, e.g. air or oxygen. This increases the temperature of the partially reformed gas, which is preferably then passed adiabatically through a bed of a steam reforming catalyst, again usually nickel on a suitable support, to bring the gas composition towards equilibrium.
The initial steam reforming stage may be carried out in one or more stages of adiabatic low temperature steam reforming, where the hydrocarbon/steam mixture is passed adiabatically through a bed of steam reforming catalyst in a process known as pre-reforming, or the steam/hydrocarbon mixture may be passed through externally-heated tubes containing a steam reforming catalyst in a heat exchange reformer in a process known as primary reforming. If necessary, the pre-reformed gas stream may be subjected subsequently to primary reforming. Where the feed gas to the partial combustion stage is a hydrocarbon/steam mixture or a pre-reformed feed gas, the subsequent partial combustion/steam reforming process is known as autothermal reforming and where the feed gas is a primary reformed gas, the subsequent process is known as secondary reforming. The principal differences between the autothermal and secondary reforming processes are the composition, e.g. the hydrogen content, and temperature of the partially reformed gas fed to the partial combustion step. Typically a pre-reformed gas fed to an autothermal reformer will contain less than 10%, no more than 20% by volume hydrogen and be at a temperature less than 650° C. whereas a primary reformed gas fed to a secondary reformer will contain greater than 10% hydrogen by volume and be at a temperature greater than 650° C. Autothermal or secondary reforming serve three purposes: the increased temperature resulting from the partial combustion and subsequent adiabatic steam reforming results in a greater amount of reforming so that the reformed gas contains a decreased proportion of residual hydrocarbon (methane). Secondly the increased temperature favours the reverse shift reaction so that the carbon monoxide to carbon dioxide ratio is increased. Thirdly the partial combustion effectively consumes some of the hydrogen present in the steam-reformed gas, thus decreasing the hydrogen to carbon oxides ratio. In combination, these factors render the autothermal/secondary reformed gas formed from natural gas as a feedstock more suited for use as synthesis gas for applications such as Fischer-Tropsch synthesis than if the autothermal/secondary reforming step was omitted. Also more high-grade heat can be recovered from the autothermal/secondary reformed gas: in particular, the recovered heat can be used to heat the catalyst-containing tubes of the primary reformer. Thus the primary reforming may be effected in a heat exchange reformer in which the catalyst-containing reformer tubes are heated by the secondary reformed gas. Examples of such reformers and processes utilising the same are disclosed in for example U.S. Pat. No. 4,690,690 and U.S. Pat. No. 4,695,442.
Fischer-Tropsch processes produce hydrocarbons from the synthesis gas stream. Water is a co-product in the reaction, which may be described as follows;nCO+2nH2→(CH2)n+nH2O
We have found that the efficiency of such hydrocarbon synthesis processes may be improved by utilising at least a portion of the co-produced water from the Fischer-Tropsch process in a saturator to provide steam for the steam reforming process. Moreover, the co-produced water from a Fischer-Tropsch process can contain significant quantities of oxygenated hydrocarbons such as alcohols, aldehydes, ketones and carboxylic acids. These give rise to a need for subsequent waste-water treatment. By returning Fischer-Tropsch co-produced water to the reforming process, the present invention advantageously returns the oxygenates to the reforming process as a source of hydrogen and carbon oxides and also reduces the need for waste-water treatment.